The present invention concerns integrated circuit devices comprising multiple active devices and passive elements on a single die and, in particular, a novel structure and method for determining the degree of isolation between devices and elements.
Monolithic integrated circuits, including both analog and digital circuits, include a multiplicity of both active devices and passive elements formed on a single die. For such circuits to be effective, it is essential that certain structures be electrically isolated from each other over a wide frequency range. For example, monolithic microwave integrated circuits, also known as MMICs, may include a number of transistors with isolated collectors formed on a p-type silicon substrate with epitaxial n+ and n-type silicon layers thereon.
Isolation in the integrated circuit art is typically achieved by forming vertical grooves, or trenches, which may be one-half to two microns wide and six to eight microns deep. Selective interconnection between isolated structures is then accomplished by technique known in the art as "air bridging". An alternate interconnection technique that is finding increasing favor in the art because of its compatibility with planar processing and multi-dimensional structures involves "back filling" of trenches with a suitable dielectric material.
In any event, the effectiveness of the isolation in the prior art has generally been determined by a DC measurement, which fails to predict behavior either at high frequencies or in digital applications, or more commonly, from an assessment of a circuit's performance after complete fabrication, often necessitating costly empirical optimization. Therefore, it is desirable for the designer of both the integrated circuit and the fabrication process to have a quantitative measure of isolation. Such a measure of isolation at the frequencies of interest would permit the selection of materials, dimensions and processing steps for isolation more quickly, more effectively and at lower cost than previously. Moreover, such measure should be capable of being incorporated as a wafer scale quality control tool.
Accordingly, an object of this invention is structure and method for quantitatively determining the electrical isolation between regions on a semiconductor substrate.